As a device to measure magnetism emitted from a living body, a biomagnetism measurement device using SQUID (Superconducting Quantum Interference Device) sensors has been researched (Patent Documents 1 to 5, for example). By arranging a large number of SQUID sensors and using the SQUID sensors to measure biomagnetism, two-dimensional magnetism information such as magnetoencephalograms or magnetocardiograms can be obtained.
In order to measure biomagnetism with SQUID sensors, the SQUID sensors need to be maintained in a superconductive state with a refrigerant. Hence, SQUID sensors are placed in a Dewar flask where a refrigerant is stored and are used for the measurement in a state in which the SQUID sensors are soaked in the refrigerant.
There has been proposed a biomagnetism measurement device in which a portion of the outer wall part of a refrigerant tank of a Dewar flask is formed in a shape fit for a measurement target part of a living body such as a skull and many SQUID sensors are arranged on the inner side of the outer wall part in such a way as to be soaked in a refrigerant. The biomagnetism measurement device can obtain a magnetoencephalogram or the like by touching a living body with the outer side of the outer wall part and accordingly bringing the many SQUID sensors close to the living body with a certain distance therebetween and by performing the measurement.
By the way, in order to detect weak biomagnetic signals, it is necessary to remove a large external magnetic field(s) and detect the signals.
While the signal strength of biomagnetic signals is 10−10 T order to 10−15 T order, large external magnetic fields (coarse magnetic fields) are very strong; for example, magnetic noise in a city is 10−7 T order and geomagnetism is 10−5 T order. If biomagnetic signals derived from a living body and a coarse magnetic field are sensed at the same time, the signals derived from a living body are drowned out by the coarse magnetic field.
Hence, in order to perform the measurement in an environment where no coarse magnetic field exists, a magnetism shielded room or the like is proposed.
Further, in order to remove external magnetic fields, a gradiometer is proposed for a biomagnetism detection device using SQUID, for example.
The gradiometer has two pickup coils arranged with a space therebetween and is described, for example, in Patent Documents 6 to 8.
Although the magnetism shielded room can shield large external magnetic fields (coarse magnetic fields), as with a magnetic field emitted from a heart being noise in neuromagnetic field measurement, a magnetic field emitted from a living body could be noise for biomagnetic signals of a detection target. Thus, the magnetism shielded room may be an insufficient measure.